Previous work has shown that purified murine alpha-macroglobulin (AMG) isolated from both tumor ascites (T-AMG) and normal plasma (N-AMG) is suppressive to mitogen induced lymphocyte transformation, the mixed lymphocyte reaction, and tumor cell growth in vitro. The proposed study seeks to define in precise biochemical terms the mechanism of inhibition of mouse AMG on neoplastic cells. An operating hypothesis assumes a potentially important role for the protease binding property of this macroglobulin. This property of the murine protein is shared with human alpha2-macroglobulin but these two proteins do not cross react antigenetically. Further comparison of the two proteins will include amino acid composition, peptide mapping and automated amino acid sequence analysis. Attempts will be made to identify components of tumor cells that interact with the murine AMG.